In many diagnostic situations it is desirable for a doctor to be able to monitor a patient's blood pressure over long periods of time. Numerous blood pressure monitoring systems have been developed to provide automatic measurement of a patient's blood pressure. Many of these automatic monitoring systems are based on oscillometric techniques in which an indirect indication of blood pressure is obtained by measuring pressure variations in a pressurized cuff placed on the patient's arm. Such measurements can be made during either the pressurization or the depressurization of the cuff. The cuff pressurization cycle is commonly referred to as the upramp, while the depressurization cycle is referred to as the downramp.
It is extremely desirable to have a linear pressurization or depressurization curve when using an automatic monitoring system. Linearity of the curve significantly decreases the lenght of time required for measurement and allows the system to make a more accurate measurement. In addition, there are certain advantages in measuring blood pressure on the upramp portion of the cuff pressurization curve. In particular, a system which measures on the upramp avoids the need to inflate the cuff to an unnecessarily high level which can cause discomfort for the patient. For systems making measurement on the upramp portion of the cuff pressurization curve, it is also desirable to have the cuff quickly pressurized to a predetermined level, e.g., 40 millimeters of mercury, to avoid making unnecessary measurements at pressure levels well below those at which systolic and diastolic pressure are expected to occur.
One type of cuff inflation system used for automatic blood pressure monitoring devices employs a source of pressurized gas, such as a conventional carbon dioxide canister. The flow of the pressurized gas can be controlled by appropriate valves and expansion chambers to provide a substantially linear cuff pressurization curve. Another approach to providing a cuff inflation system involves the use of an electric pump which is controlled by a closed-loop, pressure feedback system to obtain the desired inflation profile over the range of pressures which are used for measuring the oscillometric pulses. Although electric pumps can be controlled to produce the desired inflation profile, such pumps produce pressure waves and acoustic noise. The pressure waves propagate through the pneumatic system and interfere with the detection of oscillometric pulse data. The acoustic noise tends to be an annoyance for the user, since the measurement system typically is used over long periods of time.